The present invention relates to a device for determining the diameters of a crystal that is pulled from a liquified material and method for accomplishing the same.
Such a device is used, for example, for measuring the diameter of crystals when pulling single crystals in accordance with the Czochralski method.
In the field of crystal growth, a number of different methods are known; e.g., crystal growth from a gas phase, from a solution, or from a liquified material. The various methods for crystal growth from a liquified material have attained pre-eminence among the growth methods due to their highly developed processing technology and their production quantity.
The best known methods for crystal growth from a liquified material are the Kyropoulus method, the Bridgman method, and the Czochralski method. In the Kyropoulus method, a cooled seed crystal is dipped into a liquified material. In the Bridgman method, a crucible is lowered vertically into a temperature gradient. In the Czochralski method, a crystal is pulled from a liquified material.
With the Czochralski method the original material melts in a crucible as is also the case with the Kyropoulus method. A seed crystal is submersed in the liquified material and is wetted by it and, in so doing, fused. Then the seed crystal is pulled continuously upwards out of the liquified material while the temperature is lowered. In so doing, the crystal and crucible rotate counter-current. The speed of drawing it and the temperature of the liquified material are controlled in such a way that the crystal grows with a constant diameter after developing a shoulder. The orientation of the growing crystal corresponds to the seed crystal. This procedure is known in the art. See for example, Bonora, xe2x80x9cCzochralski Growth of Single-Crystal Siliconxe2x80x94A State-of-the-Art Overview.xe2x80x9d Microelectronic Manufacturing and Testing (September 1980), pp. 44-46 the entire disclosure of which is relied on and incorporated herein by reference.
The target diameter of the single crystal pulled in production today is geared toward the wafer size processed in semiconductor technologyxe2x80x94a size that has been taking on larger and larger values due to reasons of economy in spite of the advanced miniaturization of the electronic structural components, and thus today it is predominantly at 150 to 200 mm. There are, however, plans for wafers with a diameter of 300 to 400 mm. Given these dimensions the crystal structure and purity and especially the regularity of the diameter along the cylinder-shaped crystal play an important role in a flawless single crystal. The smoother the cylinder wall is, the smaller the expected expenditure on processing and the loss of materials. For this reason controlling the diameter during the target method is an important criterion for economy.
In practice, one comes up against considerable obstacles when trying to determine and control exactly the actual diameter of the crystal in all phases of the growing process.
To overcome these difficulties, mechanical, electrical, and optical solutions have already been proposed.
In the case of a mechanical solution, the weight of the crystal is monitored and the diameter is inferred from this weight (GB-PS 1 457 275). In so doing, a signal is produced that corresponds to the effective inert mass of the crystal when pulled out. In each case this signal is compared to the calculated expected value. If the two signals deviate from each other, the pulling speed is changed to match the actual crystal""s diameter to the target diameter via a control system intervention. A disadvantage of this method is that it is subject to various uncertain interferences as a result of the slow crystal growth.
In a refinement of this solution, a method is proposed with which the effect of the heat delay is compensated for during the crystal formation (DE-OS 25 13 924).
Another known solution to the problem of measuring the diameter of a crystal pulled out of a liquified material based on mechanical principles makes use of the torsional moment that occurs because of the relative rotation between the crystal and the liquified material; see DE-OS 36 40 868.
Measuring the diameter of crystals with the help of an electrical method is also already known (DD-PS 145 407). In this connection the electrical resistance of the growing crystal is measured while the DC or AC voltage flows through the crystal or through the system of the heatable crucible, liquified material, crystal, and pulling objects. To measure the electrical resistance of the crystal, a floating contact area, which does not effect a reaction with the liquified material or influence the thermal conditions of the boundary surface between the liquified material and the crystal due to the specificity of its material and its structural peculiarities, is located on the surface of the liquified material.
In the case of another known method for pulling single crystal rods with a uniform diameter from a liquified material contained in a crucible, optical agents are used to measure the crystal diameter (DE-PS 16 19 969). In so doing, changes in the rod diameter are constantly balanced out by using a control system that consists of mechanical servo components and one or several emission detectors that send emissions onto the liquified material. The emission detectors are adjusted in such a way that they capture the emission energy produced by a small surface area of the liquified material directly near the growing crystal in the near infrared and the visible spectral region, and its optical path and the crystal axis form an acute angle.
Also known is an optical method for measuring the diameter of a semiconductor rod produced through zone melting. By this method, the rod is filmed by a TV camera in the area of the zone melting, the camera signal is transformed into a binary video signal by comparing it with a variable threshold value, and the diameter of the rod is measured at the site at which a jump in brightness is determined that characterizes the solid-fluid boundary that occurs in the axial direction (Journal of Crystal Growth, 13/14 (1972), pp. 619-23).
In an improvement of this method the site of the phase transition between the liquified material and the semiconductor crystal growing out of it is determined more accurately by taking more pictures with different threshold values and by examining the video signals obtained with the various threshold values to see whether a zone of a prespecified minimum width exists that extends over the rod cross section and that is darker than a neighboring take-off area (DE-OS 33 25 003).
The precise determination of the actual diameter of a crystal by using the optical method is, however, subject to various interferences during the growth process that can falsify the results in such a way that carrying out the method accurately is no longer possible. As a result, the quality and results of the growth process can be strongly jeopardized. Included as interferences are, among other things, strongly variable brightness and contrast ratios on the objects to be measured; i.e., on the crystal, the liquified material, or the luminous meniscus ring around the crystalxe2x80x94and interfering reflections on the liquified material or the unsteadiness of the object to be measured caused by mechanical interference.
Moreover, to a certain extent the geometrical form of the corrected crystal can deviate significantly from the ideal form of a cylinder with a circular cross section. By varying the crystal diameter, visibility on the entire diameter of the crystal and the luminous meniscus ring belonging to it is considerably limited. Moreover, the components and devices for optimizing the temperature distribution limit the visibility of the crystal and further jeopardize it.
Even with the present normal crystal diameters of about 150 to 200 mm these problems with the detection and control of the crystal diameter can lead to considerable disadvantages with the growth process. For the future generation of 300- to 400-mm crystals, the problem of the reliable control of the crystal diameter will continue to intensify.
Also known is an optical system or process for controlling the growth of a silicon crystal in which the aforementioned problems are solved and in which the diameter of the silicon crystal is measured with the help of a TV camera, whereby the surface of the liquified material presents a meniscus, which is visible as a light area near the silicon crystal (EP 0 745 830 A2). In this system, first a test pattern of a part of the light area near the silicon crystal is photographed by the camera. Then the characteristics of the test pattern are detected. A valid characteristic of a test pattern is, for example, the intensity gradient. After this an edge of the light area is defined as a function of the detected characteristics. Then a contour, which includes the defined edge of the light area, is defined, and finally the diameter of the defined contour is determined, whereby the diameter of the silicon crystal is determined as a function of the desired diameter of the defined contour.
A disadvantage with this system is that the accuracy in some applications is not yet high enough and, more particularly, external interferences are not sufficiently taken into consideration.
Moreover, a device and method for the pulling of single crystals according to the Czochralski method, in which the diameter of a crystal is determined by an evaluation control unit, is known (DE 195 48 845 A1). In this connection two cameras are provided whose picture axes form an angle of 90 degrees. With these cameras, different points are detected on the same object; e.g., a meniscus ring. The focal distance and the image angle of these cameras are identical, as a result of which it is not possible to take a picture of areas of different sizes of an object on the picture plane while guaranteeing that at least one coordinate is completely filled.
Finally, there is a transition-angle identification system in which two cameras can be used whose picture axes form an angle in a vertical plane of a crystal (U.S. Pat. No. 4,943,160, FIG. 4). By the term transition angle one is to understand an angle that forms a meniscus surface with the vertical axis of a crystal and with an axis parallel to this vertical crystal axis. Other areas of the crystal are not picked up by the camera. The cameras also do not have a focal distance or image angle that is adjusted to the diameter of various crystal parts.
Therefore, an object of the present invention is to determine the diameter of a crystal with great precision by using an optical detection system.
In achieving the above and other objects, one feature of the invention resides in a device and process for the determination of the diameters of a crystal that is pulled from a liquified material. In this connection a plurality of video cameras are provided, each of which reproduces its own section along the vertical axis of the crystal or in a direction vertical to it. The image angles of the camera are laid out in such a way that the object to be reproduced completely fills the entire picture plane, at least in one direction. For objects with a small diameter, for example, the crystal neck, a camera with a small image angle is used. For objects with a large diameter, as for example, the crystal body, a camera with a large image angle is used.
More particularly, the present invention features a device for determining the diameters of a crystal that is pulled from a liquified material, employing a plurality of cameras for the reproduction of a corresponding number of areas of the crystal, and where the image angle (xcex3, xcex4) of the plurality of cameras are matched to each of the areas reproduced by them.
In further detail, it is a feature of the present invention that the device for the determination of diameters of a crystal by using a plurality of cameras, the cameras are used one after another, but during the same process.
According to a still further feature of the present invention, the above and other objects are achieved by a process for the determination of the diameter of a crystal that is pulled from a liquified material, comprising carrying out the following steps using the evaluation control unit:
a) correction of the picture of the crystal for problems that arise from being taken at a slant so that, once the calibration is taken into consideration, the elliptical form of a crystal circumference in the reproduction is converted into a circle,
b) scanning of the circumference that has been converted into a circle for its diameter, whereby the two corner points, which mark the diameter, are detected,
c) searching for a third point on the circle,
d) calculating whether the two corner points and the third point lie on a common ideal circle.
The advantage realized by the invention consists, in particular, of the fact that the camera resolution can be adjusted to each measurement problem by using at least two cameras. In the case of large crystal diameters the resolution can be doubled, for example, by using two cameras that make a composite picture possible. In addition, the problems that occur due to varying light strengths and contrast ratios can be eliminated. By taking into consideration the different relevance conditions for valid measurements, a stable signal and consequently an improved method of carrying out the process are also obtained. In addition to this, an automatic absolute-value calibration is possible, and the crystal diameter can be measured in such phases of the process in which a meniscus does not occur.